Due to its properties, silicon can convert only light with a wavelength in the range of 400 nm to 1100 nm to photocurrent that is output. The reason is as follows: the photo energy of light with a wavelength of 1100 nm or longer is lower than the energy band gap of silicon (1.12 eV), so it cannot generate electron-hole pairs. Also, short-wavelength light can generate electron-hole pairs only near the silicon surface. Usually, on the surface of silicon, the recombination rate is very high, so the electron-hole pairs generated under light at a wavelength of 400 nm or shorter immediately recombine and the photo-carriers are annihilated in the silicon.
For a PIN photodiode, there are two important characteristics, that is, the sensitivity (photoelectric conversion efficiency) and the BW (response speed). Based on the basic principle of a PIN photodiode, FIG. 2 shows the cross-section of a PIN photodiode with a constitution in consideration of said two important characteristics with respect to the wavelength of blue light (λ=405 nm).
For PIN photodiode (100) shown in FIG. 2, on high-concentration p-type single crystal silicon substrate (110), low-concentration p-type silicon layer (112) is formed by means of epitaxial growth, low-concentration n-type silicon layer (114) is formed by epitaxial growth, and intermediate-concentration n-type silicon region (116) having a prescribed depth from the surface of silicon layer (114) is formed. When a reverse bias voltage is applied, a depletion region is formed that spreads up/down from the joint surface between silicon layer (112) and silicon layer (114).
Said silicon region (116) is defined by field oxide film (118). By means of thermal oxidation, silicon oxide film (120) is formed on said silicon region (116), and silicon nitride film (122) is formed on silicon oxide film (120). Said silicon oxide film (120) and silicon nitride film (122) form an anti-reflection film.
On the two end portions of n-type silicon region (116), high-concentration n-type contact region (124) is formed, and said contact region (124) is electrically connected via platinum silicide layer (126) (PtSi) to electrode (128). Also, high-concentration p-type contact region (130) is formed via n-type silicon layer (114) and extending into p-type silicon layer (112). Said contact region (130) is electrically connected via platinum silicide layer (132) (PtSi) to electrode (134).
In addition, on the silicon substrate, multi-layer wiring region (136) and protective film (138) are formed. On multi-layer wiring region (136) and protective film (138), opening H is formed for exposing silicon nitride film (122). Said opening H defines the light-receiving surface of the PIN photodiode. When a reverse bias voltage is applied on electrode (128) on the cathode side and electrode (134) on the anode side, a depletion region is formed from the interface between silicon layer (112) and silicon layer (114). The depletion region almost reaches the silicon surface region, and, when blue light is incident on opening H, electron-hole pair carriers are generated in the depletion region. Here, the electrons move to electrode (128) on the cathode side, and the holes move to electrode (134) on the anode side. As a result, a photocurrent is output.
For a PIN photodiode, as for the photodiode shown in FIG. 1, if an electrode is not present on the light-receiving surface, a decrease in the light quantity incident on the depletion region can be suppressed. On the other hand, if an electrode is not arranged on the light-receiving surface, the movement distance of the carriers generated in the depletion region near the silicon surface increases, and the proportion of annihilation by recombination becomes high. Especially, if plural silicon unbonded bonds (dangling bonds) are present on the surface of the silicon, the carriers are trapped in the trap level of the silicon interface, and the probability of annihilation of the carriers by recombination increases. When silicon oxide film (120) is formed by thermal oxidation on silicon region (116), the number of dangling bonds of silicon decreases and the interface trap phenomenon can be suppressed to a minimum level.
In addition, if the resistance of the silicon surface is high, the carrier movement velocity falls, and the response speed decreases. However, intermediate-concentration n-type silicon region (116) is formed at a prescribed depth from the surface of n-type silicon layer (114), so carriers generated near the silicon surface move through silicon region (116) near the low-resistance silicon surface, and the decrease in response speed can be suppressed.
However, for the photodiode shown in FIG. 2, a wafer with a problem of low sensitivity is generated. Here, the problem of poor sensitivity occurs when light at a wavelength of 405 nm (blue-violet light) is incident.
The present inventors have concentrated on the cause of generation of the problem of poor sensitivity with regard to the silicon-oxide film interface at the surface of a photodiode and that most significantly affects the sensitivity when light at a wavelength of 405 nm is incident. With regard to the wavelength of light and the properties of silicon, light at a wavelength of 405 nm can reach only the vicinity of the surface of silicon, so the generated photo carriers are significantly dependent on the silicon surface state. FIG. 3A shows the relationship between light absorptivity and the depth from the silicon surface by means of relationships with the wavelength at 410 nm, 660 nm and 780 nm, respectively. The wavelength at 410 nm can reach only a depth of about 1×10−6.
Consequently, if many dangling bonds of silicon are present on the silicon surface, the carriers generated under incident light at a wavelength of 405 nm are trapped on the dangling bonds and are annihilated during the period of movement when the carriers are attracted by the electric field to move to the electrodes, so conversion to and output of photocurrent does not occur. This is one of the reasons for the problem of poor sensitivity of a PIN photodiode.
For the photodiode shown in FIG. 2, since the silicon oxide film is formed as said thermal oxide film, dangling bonds are minimized, and the problem of poor sensitivity can be alleviated. However, due to dispersion in the manufacturing process and stress in reliability testing, silicon dangling bonds are still generated, and said problem of poor sensitivity at 405 nm occurs.
The objective of the present invention is to solve the aforementioned problems of the prior art by providing a manufacturing method for a semiconductor device containing a photodiode characterized by the fact that it has stable high sensitivity for short-wavelength light near 405 nm.